This invention relates to a method and apparatus for measuring the volume of an object by liquid displacement. In particular, this invention relates to the measurement of the increase in volume of the paw of an animal, the increase due to the inflammation of the paw. An instrument of this type has been described as a plethysmometer.
The present invention may be used in the so called, rat paw edema assay, the rat adjuvant arthritis assay and functionally related models. An adjuvant arthritis model is illustrated below. The, so called, rat paw edema assay, is a classic assay used to assess the ability of a putative anti-inflammatory agent to prevent the edema that would otherwise have been produced by the injection of a pro-inflammatory agent. See Winter et. al., Proc. Soc. Exp. Biol. Med., 111, 544 (1962). WO 95/18799, published Jul. 13, 1995 describes one such assay as follows:
“Male Sprague-Dawley rats (150-200 g) were fasted overnight and were given to either vehicle (5% Tween 80 or 1% Methocel) or a test compound at 9-10 a.m. One hr later, a line was drawn using a permanent marker at the level above the ankle in one hind paw to define the area of the paw to be monitored. The paw volume (VOh) was measured using a plethysmometer (Ugo-Basile, Italy) based on the principle of water displacement. The animals were then injected subplantarly with 50 μl of a 1% carrageenan solution in saline (FMC Corp., Maine) into the paw using an insulin syringe with a 25-gauge needle (i.e. 500 g of carrageenan per paw). Three hr later, the increases in paw volume (V3h−VOh) were measured.”
Illustrative of plethysmometers sold for purposes of conducting such assays are those sold by Buxco, Electronics; Muromachi Kikai Co., Ltd.; and Ugo Basile.
Historically applicants used an in-house system that was developed in the 1970's and in the 1990's purchased a commercial unit from Buxco®. Both of these methods for determination of paw volume changes are based on fluid pressure measurement. They are more difficult to use, prone to sensor drift, and susceptible to errors caused by trapped air bubbles or improper priming.
This application presents a new method for paw volume determination, and compares the accuracy of this system to a representative reference system.